Disaster Recovery in an Uncertain World

Following the September 11 terrorist attacks, the storage-networking industry found itself in an awkward situation. Storage Area Network (SAN) advocates have always promoted disaster recovery. Use of Dense Wavelength Division Multiplexing (DWDM) or new IP storage technologies to extend the reach of storage beyond primary data centers have enabled remote mirroring and remote backup of vital corporate data. Fortunately for the collective karma of the storage-networking industry, few vendors have attempted to exploit the surge of interest in disaster recovery. Instead, most vendors have simply restated the technology benefits that SANs generally provide for disaster recovery, highlighting new functionality that can enhance design and implementation of business-continuance strategies.

As with tape backup in general, disaster recovery hasn't been a favored career path for storage administrators. Safeguarding ever-changing corporate data is often a monotonous routine that, like paying insurance premiums, administrators do grudgingly but on a fixed schedule. Although the development of larger tape libraries and higher-capacity tape cartridges has assisted backing up vital corporate data to tape, the primary issue is not scheduling or tape rotation, but restoring data from tape in the event of failure. Restoration imposes a lengthy downtime during which a company might lose ongoing business. Consequently, despite the higher cost of disk mirroring to maintain a readily accessible copy of data, companies are increasingly deploying mirroring strategies as part of an optimum disaster-recovery solution.

Intelligent controllers in higher-end storage devices offer new options for disk mirroring, including the ability to have two separate storage units use each other as secondary mirrors. For disaster recovery implementations, secondary mirroring lets two geographically separate data centers provide mutual support if either center suffers failure, performing on a grand scale the failover capability of server clustering. Mirroring between two remote sites, however, isn't a trivial undertaking, especially if it involves long distances. Depending on the applications that the systems run to storage, the amount of data that mirroring must synchronize between two locations might require sustained gigabit bandwidth in both directions. Initial attempts to use DWDM for extending distance were successful within a metropolitan area, but DWDM hasn't scaled well to regional distances. For example, systems can use DWDM for simple SAN extension between New York and New Jersey. Mirroring between New York and Chicago, however, is problematic.

Unfortunately for us all, no one knows the distances our infrastructures now require to safeguard human and corporate data resources. Mirroring between two buildings in the same city seemed sufficient previously, but now we realize that an entire city might be a victim of direct or indirect disruption. Consequently, industry consultants are now recommending business-continuance solutions that span regions, maintaining mirrors that might be separated by hundreds or thousands of miles.

Businesses situated in geologically unstable areas, such as California or the Pacific Northwest, established the precedent for long-distance mirroring. Until recently, achieving stable, high-performance links between regions has been difficult. Carriers such as Qwest Communications International now offer gigabit and multigigabit services that can span coast to coast. For storage applications such as remote mirroring, new IP storage technologies have demonstrated sustained gigabit performance over thousands of miles. With the new disaster-recovery strategies that these solutions enable, the business world can concentrate on the more crucial issue of preserving resources in the event of catastrophe.

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